The detection of defects in underground or underwater pipelines is a problem which has been solved in part by the use of an inspection device using photographic apparatus such as disclosed in the O'Connor et al U.S. Pat. No. 4,373,658, issued Feb. 8, 1983. That device has a number of very important advantages over previously proposed inspection devices having photographic apparatus or other types of defect detecting apparatus, including ultrasonic systems. However, the use of photography has an important limitation when inspecting long lengths of pipeline in that there are limitations on the amount of film which can be carried and there are also limitations in the energy which is available, especially when batteries are used as the energy source.
With regard to ultrasonic inspection systems, they are well known but have not been used to any substantial extent in internal pipe inspection because of recognized limitations thereof. In a typical pulse-echo type of system, an electronic signal generator is provided which generates pulses or periodic wave trains and a sending transducer responds thereto to emit a burst of ultrasonic energy. A couplant is required to transfer energy from the sending transducer to the test piece. A receiving transducer is provided to receive and convert energy reflected back on the interior of the test piece and develop corresponding electrical signals. In many cases, the same transducer is used as both a sending transducer and a receiving transducer. When a separate receiving transducer is provided, a couplant is required between it and the test piece. A display or indicating device, typically a cathode ray tube, is associated with the detector means to produce indications of reflections from internal flaws in the test piece.
Such systems have many advantages but have the well known limitation that a liquid or solid-contact coupling is needed to provide effective transfer of ultrasonic energy between transducers and the parts being inspected. A liquid or solid-contact coupling is needed because sonic waves are highly attenuated in a gaseous medium, especially at higher frequencies, and because sonic waves are almost completely reflected at metal-gas interfaces, whereas only partial reflection occurs at metal-liquid or metal-solid interfaces and sufficient energy may be transmitted for detection of defects within the part. It is apparently because of this well known limitation of ultrasonic systems that they have not been used to any substantial extent in internal pipe testing, it being noted that it is very difficult to provide a liquid or solid-contact coupling between an ultrasonic transducer and the inside of a pipe, especially when the transducer is to be moved along the pipe and when the pipe may have weld beads or other protrusions or irregularities in its inside surface.